The propagation of high energy laser beams in the atmosphere is subject to turbulent disturbances that are caused by temperature fluctuations, and thus minor fluctuations in the refractive index of the air, and vary with time. These disturbances result in a local deflection of the high energy laser beam (the so-called tip/tilt portion) and an additional non-linear change in the beam profile. The effect of the high energy laser beam on the target deteriorates if the radiation spot does not constantly remain at the same location on the target object (even if the latter moves) but instead moves around on the target object. If the power of the high energy laser (HEL) is to be maximized on the target object, it is necessary to be familiar with and compensate for the turbulent disturbances. To this end, the target must initially be observed, for which purpose normally an image acquisition device is provided that detects light emitted or reflected by the target object.
In the case of extended targets, the following effects lead to an impairment in the target object image recorded by the image acquisition and thus to potential worsening of the stabilization of the radiation spot on the target object:                Non-homogeneous temporal variation in the surface brightness of the target object observed by the image acquisition device, e.g. moving glints, non-homogeneous illumination, shadows, effects of turbulence, etc.;        When the target is illuminated with an illuminating laser, additional speckle effects and non-homogeneities in the surface brightness may occur;        Heavy blurring of the target contours, e.g. by turbulences, non-homogeneous target illumination conditions, and speckle effects.        
Moreover, the turbulence in the air that the high energy laser beam passes through on its way to the target is subject to local fluctuations; for the observability of the turbulence it is therefore necessary to detect these fluctuations within limited areas around the high energy laser beam. Typically the Fried parameter r0 or the isoplanatic angle Θ0 is used for evaluating the locally suitable areas. They describe local or angular areas outside of which a significantly changed turbulence may be assumed.
Conventional ideas for detecting turbulence are known—for instance, using guidestars in astronomy or tip-tilt laser spots in connection with high energy lasers when detecting the tip-tilt portion. So-called adaptive optics are used for detecting higher modes of turbulence. These devices for detecting turbulence in accordance with these known methods each require additional sensors and/or transmitters.
In the field of image processing, widely used methods are measuring the center of gravity of the image of the target object in the image acquired by the image acquisition device or alternatively evaluating a four quadrant detector.
The drawback of this is that the temporal variations in the surface brightness are interpreted as a virtual turbulent tip-tilt movement and produce additional noise that has a significantly detrimental effect on the stabilization of the radiation spot on the target object. Image blurs also lead to additional noise.
Alternative methods, for instance correlation methods, are also sensitive to strong fluctuations in surface brightness. Contour tracking methods are less sensitive to target surface fluctuations in brightness, but produce significant additional noise in the image acquired by the image acquisition device with contour blurs of the target. Statistical methods published in the literature attempt to estimate turbulent image degradation, but do not solve the problem of temporal variation in surface brightness.